A lot of different spray drying processes and equipment therefore have been developed during the last many decades. A standard textbook on this technology is Masters, Keath: Spray Drying Handbook, 5th edition, Longman Scientific & Technical (1991), incorporated herein by reference.
It is conventional to select the spray dryer design and configuration and also the process parameters in consideration of the type of product to be dried and the desired characteristics of the final product, e.g. agglomeration, particle size, density etc.
Some of the issues hitherto considered in this respect are drying chamber design as to shape and dimensions; integration of a fluidized bed in the chamber bottom; integration of filters for separating product from the drying gas; selection of type of atomizer for the feed --rotary atomizer or nozzles, pressure nozzles or 2-fluid nozzles; type of gas disperser; drying gas temperature and velocity; feed-spray and gas flow directions; feed formulation and properties etc.
Other means for influencing product characteristics comprise separation of the total drying process into two or more steps in which the temperatures are controlled individually, recirculation of fine particles as well as control of several other parameters.
However, in spite of the fact that numerous measures are thus conventional for influencing product characteristics there is still room for improvements within certain areas of spray drying technology.
Thus, spray drying of some products involves creation of large vacuols in the droplets during drying thereof which results in blowing-up "balloon" particles, having thin walls which may break down before the drying process is terminated. Such breaking down of the particles results in a low-density and dusty product implying disadvantages in handling, transport and use, e.g. as pharmaceuticals.
Certain pharmaceuticals are preferably administered in formulations in which they are present in amorphous state. This is due, e.g. to the fact that the solubility rate for these pharmaceuticals is higher for the amorphous form than for crystalline forms thereof. Several modern pharmaceuticals have such low solubility rates in crystalline form that their bioavailability after administration is impeded thereby. Therefore, there is a need for preparing such pharmaceuticals with a structure wherein the amorphous state is more dominating than in the structure obtained by the conventional spray drying methods. The preference of pharmaceuticals in amorphous form is described inter alia in WO 98/57967 A, U.S. Pat. Nos. 5,612,367 and 5,641,745.
The amorphous form may be the preferred one in several forms of pharmaceutical preparations intended for various routes of administration.
The difficulties in obtaining a dominating amorphous structure when spray drying certain products are to some extent connected to the creation of thin-walled easily breaking particles since the surfaces exposed by breaking of said walls may initiate or accelerate crystallization processes.
Apart from the above described problems connected to the obtainment of a low-density product consisting to a large degree of fractured particle walls and the problems relating to the production of a powder having amorphous particle structure, it is a problem that in conventional spray drying processes the possibility for increasing the drying rate and, thus, the capacity of a certain apparatus without impairing heat economy and product quality is very limited.